Can metallic car paint be exported in GLTF?
(Answer updated 2020-05-13)

You're looking for a feature called clear coat.  There are three relevant inputs on the Principled BSDF shader node, called "Clearcoat," "Clearcoat Roughness," and "Clearcoat Normal."

These three settings will export to glTF files beginning with Blender version 2.83 (still in Beta as I write this).  It will import from glTF files beginning in Blender 2.90 (currently in Alpha).

Clear coat is not part of the glTF 2.0 core specification, but it was added in a glTF extension called KHR_materials_clearcoat.  Not all glTF viewers can display clear coat yet, but some of the major ones have released stable versions that can, including ThreeJS, BabylonJS, and Filament.  The gltf-test project has a test of clear coat with a summary of some frameworks that support it.